That's a good question, one I don't know a direct answer to.
But I do know this:
There is a model designated as TH-42PA20B that is marketed in the UK. This model is set-up for PAL. If you look at the spec differences you will note that there is a function called "Quasi-100hz" aimed at processing PAL signals, flicker free. When this function is invoked the levels of gray scale drops back to 1024 (from 1536).
This question came up before and according to one member (Glauco ?) the American NTSC Panny (TH-42PA20u) cannot play back PAL signals. According to the literature that came with my set, it only lists as being able to display NTSC signals.
As you noted, the commercial sets are multi-signal.
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